1. Field of the Invention
This invention relates generally to an exhaust processing and heat recovery (EPHR) system and method for use with fossil fuel fired furnaces. More particularly, the present invention relates to an EPHR system in which alkaline particles are introduced into a flue gas stream to allow additional heat extraction and reduce fouling of air preheater equipment.
2. Discussion of Related Prior Art
Many power generation systems are powered by steam that is generated via furnaces fired by fossil fuels, such as, for example, coal or oil. A typical power generation system is generally depicted in the diagram shown in FIG. 1A.
FIG. 1A shows a power generation system 10 that includes a steam generation system 25 and an exhaust processing and heat recovery system (EPHRS) 15 and an exhaust stack 90. The steam generation system 25 includes a furnace 26. The EPHRS 15 may include a regenerative air preheater 50, a particulate removal system 70 and a scrubber system 80. A forced draft (FD) fan 60 is provided to introduce air into the cold side of the air preheater 50 via inlet 51. The particulate removal system 70 may include, for example, an electrostatic precipitator (ESP), and/or a fabric filter system (Bag House), or the like. Scrubber system 80 may include, for example, a wet or dry flue gas desulphurization (WFGD/DFGD) systems.
The regenerative air preheater 50 helps increase the thermal efficiency of furnace 26, thereby reducing its operating costs and emissions of greenhouse gases. An air preheater 50 is a device designed to heat air before it is introduced to another process such as, for example, the combustion chamber of a furnace 26. There are different types of regenerative air preheaters, including those that include moving or rotating heat exchange elements, such as, for example, the Ljungstrom® air preheater. Other regenerative air preheaters utilize fixed heat exchange elements and/or internally rotating hoods or ductwork that is fixed to rigid air and/or gas ducts.
FIG. 1B and FIG. 1C are diagrams generally depicting a conventional rotary regenerative preheater 50. The typical air preheater 50 has a rotor 512 rotatably mounted in a housing 524. The rotor 512 is formed of diaphragms or partitions 516 extending radially from a rotor post 518 to the outer periphery of the rotor 512.
The partitions 516 define compartments 520 there between. These partitions 516 contain heat exchange element basket assemblies 522. Each basket assembly 522 includes one or more specially formed sheets of heat transfer surfaces that are also referred to as heat exchange elements 542. The surface area of the heat exchange elements 542 is significant, typically on the order of several thousand square feet.
In a typical rotary regenerative air preheater 50, the flue gas stream, FG1 and the combustion air stream, A1, enter the rotor 512 from opposite ends/sides of the air preheater 50 and pass in opposite directions over heat exchange elements 542 that are housed within the basket assemblies 522. Consequently, the cold air inlet 51 and the cooled flue gas outlet 54 are at one end of the air preheater 50 (generally referred to as the cold end 544) and the hot flue gas inlet 53 and the heated air outlet 52 are at the opposite end of the air preheater 50 (generally referred to as the hot end 546). Sector plates 536 extend across the housing 524 adjacent the upper and lower faces of the rotor 512. The sector plates 536 divide the air preheater 50 into an air sector 538 and a flue gas sector 540.
The arrows shown in FIG. 1B and FIG. 1C indicate the direction of the flue gas stream FG1/FG2 and the air stream A1/A2 through the rotor 512. The flue gas stream FG1 entering through the flue gas inlet 53 transfers heat to the heat exchange elements 542 in the basket assemblies 522 mounted in the compartments 520 positioned in the flue gas sector 540. The heated basket assemblies 522 are then rotated to the air sector 538 of the air preheater 50. The stored heat of the basket assembly 522 is then transferred to the air stream A1 entering through the air inlet 51. The cold flue gas FG2 stream exits the preheater 50 through the flue gas outlet 54 and the heated air stream A2 exits the preheater 50 through the air outlet 52.
Referring back to FIG. 1A, air preheater 50 heats the air introduced via FD fan 60. Flue gas (FG1) emitted from the combustion chamber of the furnace 26 is received by the air preheater via inlet 53. Heat is recovered from the flue gas (FG1) and is transferred to input air (A1). Heated air (A2) is fed into the combustion chamber of the furnace 26 to increase the thermal efficiency of the furnace 26.
During the combustion process in furnace 26, sulfur in the fuel used to fire the furnace 26 is oxidized to sulfur dioxide (SO2). After the combustion process, some amount of SO2 is further oxidized to sulfur trioxide (SO3), with typical amounts on the order of 1% to 2% going to SO3. The SO2 and SO3 will be passed from the combustion chamber of the furnace 26 and into the exhaust flue as part of the flue gas FG1 that is then emitted from the steam generating system 25 and received by the inlet 53 of air preheater 50. The presence of iron oxide, vanadium and other metals at the proper temperature range allows this oxidation to take place. Selective catalytic reduction (SCR) is also widely known to oxidize a portion of the SO2 in the flue gas FG1 to SO3.
As heat is being recovered/extracted by the air preheater from the flue gas FG1, the temperature of the flue gas FG1 is reduced. It is desirable to remove the maximum amount of heat from the flue gas and transfer it to the heated air going to the furnace or the fuel pulverizer mills to optimize the thermal efficiency of the power plant. Additional heat extraction allows for the design/use of particulate collection equipment, gaseous cleanup equipment, ducting and stacks downstream of the flue gas outlet that are rated for lower temperature ranges and reduced volumetric flow rates. The lower temperature rating and lower flow rate mean that tremendous cost savings can be realized by not having to provide equipment capable of withstanding higher temperatures and higher flow rates. However, the lower flue gas temperature range may result in excessive condensation of sulfur trioxide (SO3) or sulfuric acid vapor (H2SO4) that may be present in the flue gas. As a result, sulfuric acid may accumulate on surfaces of the heat exchange elements 522 of the air preheater 50. Fly ash in the flue gas stream can be collected by the condensed acid that is present on the heat transfer surfaces. This acid causes fly ash to stick more tightly to surfaces. This “fouling” process impedes the air and flue gas flow thru the air preheater, resulting in increased pressure drop through the air preheater plus lower heat transfer effectiveness.
After a period of time, accumulations of acid and flyash on surfaces of the air preheater 50 grow so large that they must be removed in order to maintain the thermal performance and an acceptable pressure drop the air preheater. This is typically accomplished by periodically (for example, 3 times daily) “sootblowing” the heat transfer surface with compressed air or steam to remove the deposits that have accumulated on the heat transfer surface while the air preheater is operating. In addition, if required, washing the air preheater with water may be conducted during an outage of the steam generation system 25 when the furnace 26 is shut down and maintenance operations are performed.
A potential benefit to reducing the flue gas outlet temperature is that the particulate removal system 70 and scrubbing equipment 80 may be designed for a lower operating temperature. The lower temperature flue gas also has a lower volumetric flow rate. The reduction in flue gas temperature, volume and acidity reduce operating and capital costs that are associated with equipment designed for the higher volumetric flow rates, higher operating temperatures, or higher SO3/H2SO4 concentrations in the flue gas. These conditions would exist if the acid were not condensed and/or neutralized to prevent excessive fouling of the heat transfer surfaces. Once the flue gas exhaust has passed through particulate removal and scrubbing operations, it is then ready for introduction to the exhaust stack 90 for elevation and dispersion over a wide geographic area.
Extraction of heat from flue gases is beneficial and is used for performing various operations in a typical plant. However, in existing coal and/or oil fired steam generation systems, it is costly to remove additional heat from the exhaust gas stream. Excessive reduction of the flue gas temperature without consideration for the additional condensation of H2SO4 vapors in the flue gas, will result in excessive fouling of the heat transfer surfaces in the air preheater. Thus, a need exists in the industry to address the aforementioned deficiencies and inadequacies.